I don't think this would be possible. You can set a weekend from Th to Su, but it will repeat every week. Rainlendar used the current definition of 1 week = 7 days based on the Gregorian Calendar. In your case, you're talking about a week of 14 days.
Of course isn't the same (because it's an old one), but someone could ask to do the French Republican Calendar, which have weeks of 10 days.